An apparatus using an array of sensing switches to measure a liquid level is well known and one example is described in UK patent application No. 9208749.3, which has been published as PCT/GB92/01601. The apparatus described employs a vertical array of magnetic switches which are operated by means of a magnet carried by a float. Such switches have an on-off operation so that they provide stepped measurements of the liquid level, which is not necessarily a disadvantage if the steps are sufficiently small. The use of a magnetic float arrangement in some circumstances may not be desirable and so a different method is required.
Another apparatus including a capacitance sensing arrangement, which does not use a floating magnet, is also well known. These arrangements generally do not operate in a switching stepped method. Instead they measure the changes in the total capacitance of a suitable probe as it varies with the fluid level. This arrangement has the disadvantage that the capacitance measured varies with such parameters as dissolved chemical concentration, tank shape and temperature conditions. Additionally this arrangement does not produce a linear relationship and requires frequent calibration.